Permeability tuning means



Aug. 11, 1936.

w. A. SCHAPER PERMEABILITY TUNING MEANS 3 Sheets-Sheet 1 Filed June 26, 1955 5 5 M M L M W ATTORNEY.

Aug. 11, 1936. w SCHAPER 2,051,012

PERMEABILITY TUNING MEANS Filed June ,26, 1935 5 Sheets-Sheet s ll'illl INVENTOR, W/LL IAM A. SCHA PER,

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ATTORNEY.

Patented Aug. 11, 1936 UNITED STATES PEBMEABILITY TUNING MEANS William A. Schaper, Chicago, Ill., assignor to Johnson Laboratories, Inc., Chicago, ill., a corporation of Illinois Application June 26,

.6 Claims.

The invention relates to tuned high-frequency electrical systems, especially of the type intended for the reception of radio signals, and more especially to systems wherein the tuning is accom- 6 plished by variation of the inductance in each of several resonant circuits, rather than by a variation of the capacitance.

Systems tuned by inductance variations were investigated over ten years ago and several types of so-called "variometers were produced, which,

however, were entirely unsuccessful because of the excessive losses inherent in the method of inductance variation employed. More recently, and after a lapse of several years during which capacitance variation has been used exclusively,

suggestions have been made for an entirely new method of inductance variation in which a fermmagnetic core is inserted into the inductance coil. This method has been made possible by the development, also recent, of highly eiiicient magnetic quencies covers a wide range, and the tuning method must be so designed as to cover this range without resort to switching or other expedients which increase the diiiiculty of manipulating the apparatus, and also increase the cost of manufac- V ture.

It is highly desirable that the performance of a radio receiving apparatus, both as to sensitivity and to selectivity, shall be substantially constant over the range of frequencies to be received. Systerns tuned by ferro-magnetic cores have inherent advantages in this respect.

It is another object of the present invention,

therefore, to produce a new and improved method of inductance variation in which an adequate tuning range is secured without sacrifice of the inherent constancy of performance of the circuits. Additionally, the present invention provides a novel expedient for correcting those slight changes in the performance of the system which arise chiefly through mechanical limitations imposed by the commercial requirement of relatively low cost.

According to the present invention, advantage is taken of the characteristics of long solenoids, i. e.,

cylindrical windings having a relatively large ratio 1935, Serial No. 28,437

of length to diameter. Such windings have a relatively small number of layers and in many cases only a single layer. In windings of this type the inductance per turn (in the absence of a ferromagnetic core) is relatively low. This is due to 5 the fact that a considerable portion of the flux generated in one part of the winding is unable to link with other parts of the winding. When, however, a ferro-magnetic core is inserted into such a solenoid, the configuration of the magnetic field 0 is completely altered, and the number. of interlinkages is greatly increased. By this method I secure a very much greater range of inductance variation than would be possible with an inductive winding of the more conventional short sole- 15 nold type.

The invention will be better understood from the following description taken in connection with the drawing. which illustrates a typical embodiment of my invention, and in which- Fig. 1 is an elevation, partly broken away, of a complete eight-circuit tuning device embodying the invention;

Fig. 2 is a plan view of the device of Fig. 1;

Fig. 3 is a sectional view of the long solenoid 25 and cooperating ferro-magnetic core employed in the device of Figs. 1 and 2;

Fig. 4 is a diagrammatic representation of the coil and core of Fig. 3, with the core completely withdrawn from the coil;

Fig. 5 is a diagrammatic representation similar to Fig. 4, except that-it shows the core fully inserted into the coil; and

Fig. 6 is a schematic wiring diagram showing one manner in which the plural resonant circuits 35 of the device of Figs. 1 and 2 may be connected for operation.

Referring to Figs. 1 and 2, the tuning device here illustrated comprises a bed plate I upon which are mounted eight inductive windings 2. The windings 2 are supported on long insulating tubes 3. Internally of these tubes there are ferromagnetic cores 4, and mechanism is provided to move these cores into and out of the windings 2. The cores I are supported on bracket arms 5 which pass through long slots in the tubes 3. The bracket arms 5 are supported from a slide rest 6 whose motion is restrained by guide rods I. The guide rods 1 are supported from the bed plate I at one end and are maintained in parallel relation by a bracket 8 at the other end. The bracket I carries a bearing 9 through which passes a shaft Ill, which has a multiple screw-thread II. The screw-thread I I engages an anti-lost-motion Bil I! n th slide rest 6. A knob It may be provided for rotating the shaft ll, or any other suitable driving gear may be employed. The slide rest 6 is made to accurately follow-the guide rods 1 by means of a pillow H which is pressed against the guide rod I by means of a spring member l5.

Mounted upon the bed plate I and associated with the windings 2 are adjustable capacitorsfli. These capacitors are respectively appropriately connected to the winding 2, depending upon the circuit arrangement to be employed.

Referring now to Fig. 3, there is shown a twolayer banked winding. The banked winding is to be distinguished from a conventional twolayer winding. In the banked winding the two layers are wound simultaneously, that is to say, turns of the second layer are wound over turns of the first layer as rapidly as the first layer is wound, so that both layers are completed at the same time. This form of winding has the advantage that it greatly reduces the losses due to the distributed capacitance of the winding.

Due to the small diameter and considerable length of the winding 2, it is advantageous to use a two-layer banked winding for the usual broadcast range of frequencies. For other ranges of frequency a single-layer winding, or a winding having more than two layers, may be employed.

I prefer to use a stranded type of conductor for these windings, of the type commonly known as Litz. This conductor has a plurality of insulated strands and is much more eflicient for highfrequency windings than a solid conductor.

Referring now to Fig. 4, which shows the core 4 completely withdrawn from the winding 2, the magnetic field will have some such configuration as is indicated by the dotted lines. The sellinductance of the coil under these circumstances will be relatively low, because of incomplete linkage of the flux throughout the windings.

Referring now to Fig. 5, which shows the core 4 fully 'inserted into the winding 2, the flux, under these conditions, will have some such configuration as is indicated by the dotted lines in Fig. 5. This configuration is such that the greater portion of the flux lines link with all of the turns of the winding 2, and a relatively high value of inductance is secured.

The increase in inductance for the condition represented in Fig. 5, over that shown in Fig. 4, is due to two causes, namely, 1) the permeability of the space within the winding has been materially increased by the insertion of the ferromagnetic core, and (2) the linkage of the flux becomes more complete by the insertion of the core.

It will be understood that, as the core 4 is gradually inserted into the winding 2, there is a corresponding gradual change in-the configuration of the magnetic field from the condition shown in Fig. 4 to the condition shown in Fig. 5, In this way, the permeability of the space inside the winding is gradually increased by the insertion of the core and the linkage of flux is gradually increased, It is because I employ these two effects simultaneously that I am enabled to secure, with a very simple winding and core, a

relatively very large range of inductance variation.

Resonant circuits of the type here considered,

consisting, for example, of a winding 2 with a this figure two windings 2, 2 are shown and each winding has an adjustable capacitor l6 connected across it, thus forming two resonant circuits l8, IS. The resonant circuit I8 is connected across the output terminals of a thermionic amplifier 20, and the resonant circuit I! is connected across the input terminals of a thermionic amplifier 2!. In order to couple the resonant circuit l8 to the resonant circuit IS, a link circuit 22 is provided. This link circuit may consist of a relatively small number of turns wound over each of the windings 2, 2 and connected as shown. A variety of other methods of coupling the resonant circuit l8 to the resonant circuit l9 are available.

It has been pointed out by Polydorofl in U. S. Patent No. 1,982,689 that in order to secure constant performance of a resonant circuit the ratio of inductance to resistance should be maintained constant. This ratio is, to a certain extent, inherent in permeability-tuned systems, because as the core is inserted into the winding, to increase the inductance, it also tends to increase the losses. By suitably choosing a material from which the core is made, the rate oi increase of losses due to the insertion oi the iron, and therefore the rate of increase of effective resistance of the coil, can be made to correspond, at least in some degree, to the rate of increase of inductance.

In the patent above referred to, Polydorofl proposes the expedient of making anon-homogeneous core, so that the portion of the core which first enters the winding will have lower losses than the remainder of the core, to thus maintain the inductance-to-resistance ratio constant.

In my improved form or permeability-tuned circuit, employing a long solenoid and a correspondingly long homogeneous internal core, with no external portion, I obtain a much more nearly constant relation of inductance to resistance.

There is, however, still some tendency for the resistance of the winding to increase somewhat more rapidly than the inductance when the core is first introduced. This tendency can be overcome by constructing the core with slightly lower inherent losses. Such a core, however, will be relatively too low in losses when it has been completely inserted into the winding, so that the ratio of inductance to resistance will be better at the low-frequency end than it is at the highfrequency end. Since, as already pointed out, it is desirable to keep the ratio of inductance to resistance strictly constant, I introduce into the outward end of the core an appropriately tapered soft iron or mild steel pin 23 (Fig. 3). This pin is moulded into the core and, as shown, also forms the screw-threaded member by which the core is secured to the bracket arm of the motion-producing means. By appropriately shap ing the pin I can increase the losses caused by the insertion of the core to just that amount necessary to maintain the ratio of inductance to resistance constant in the desired region of the frequency band. With the winding and core proportioned as shown in Fig. 3, which is drawn approximately to scale, I obtain a ratio of maximum to minimum inductance of the order of 10 to l, which is adequate to cover the broadcast band of frequencies, or any other band of similar extent. The ratio of the length of the winding to its diameter in this case is approximately 4.25 to 1. It will be understood that by using any ratio of the order of 4 to 1, or greater, I can secure the necessary 9 to 1 inductance change.

It will also be understood that the mechanical arrangement which I have described, and which is shown in Figs. 1 and 2, is only one of a number of possible mechanisms for producing the necessary relative motion between a winding 2 and a core 4 (Fig. 3) and that any of these other mechanisms may be employed without departing from the scope of my invention.

Havingthus described my invention, what I claim is:

L-A variable high-frequency inductance device including a cylindrical coil having a ratio of length to diameter-not less than four to one, and a relatively movable magnetic core having a ratio of length to diameter not less than five to one, said variable inductance device having an efiective ratio of maximum to minimum inductance not less than eight to one.

2. A variable high-frequency inductance device including a cylindrical coil having a ratio of length to diameter not less than four to one and comprising a two-layer banked winding, and a relatively movable magnetic core of length at least equal to the length of said coil and of diameter just small enough to permit said core to be fully inserted into said coil.

3. A variable high-frequency inductance device including a cylindrical coil comprising a two-layer banked winding upon a thin-walled insulating tube, and a relatively movable mag: netic core, said coil and said core each having a 'ratio of length to diameter not less than four 5 to one.

d. A variable high-frequency inductance device including a cylindrical multi-layer bankwound coil and a magnetic core at least as long as said coil and capable of being inserted therein, said core and said coil each having a ratio of length to diameter not less than four to one.

5. A variable high-irequency inductance device having an effective ratio of maximum to minimum inductance not less than eight to one including a cylindrical multi-layer coil and an open-type magnetic core adapted to be moved into the space within said coil to produce said maximum inductance.

6. A variable high-frequency inductance device including a cylindircal coil and arelatively movable open-type magnetic core, said variable inductance device having an eii'ective ratio of maximum to minimum inductance not less than eight to one, and having at one end of said core a material for increasing the losses occurring at predetermined frequency positions.

WILLIAM A. SCI-IAPER. 

